The established art of copying holograms by contact printing into dichromated gelatin requires mounting the master hologram directly onto the new plate and exposing the combination to spatially coherent actinic light. The exposure is made with the incident, exposing light falling onto the old hologram. The incident light is partially transmitted and partially diffracted to reconstruct the incident and reference wavefronts used in the original manufacture of the hologram. These two wavefronts propagate into the new plate to produce the latent image of the copied hologram. The new plate is then processed to produce a copy of the original hologram. Incident light must be spectrally of narrow bandwidth and spatially coherent to generate and reproduce the correct interference pattern in the copy plate. As shown in FIG. 1, the exposure light 2 passes through to the surface of the glass plate master 3 containing the original hologram on the lower surface 3A thereof. Surface 3A functions as a diffraction medium for impinging light. This results in a diffracted wave entering the copy plate 4 placed beneath plate 3. Copy plate 4 has a dichromated gelatin surface 4A adjacent the surface 3A. The wave entering the copy plate 4 has two principal components--a straight through beam and a diffracted beam. When the copy plate is finished, the hologram may be read out at the original master recording wavelength.
More detailed discussion on holography and copying holograms is available in the literature. An excellent reference book is "Optical Holography" by R. J. Collier, C. B. Burckhardt, and L. H. Lin, Academic Press, 1971, pages 228-264 in particular.